HINTERLAND-VERGENT TECTONIC WEDGE BELOW THE RIWAT THRUST, HIMALAYAN FORELAND, PAKISTAN - IMPLICATIONS FOR HYDROCARBON EXPLORATION

The Riwat thrust, with a surface trace of over 50 km, is one of the ma jor faults in the footwall of the main boundary thrust in the Himalaya n foreland of Pakistan, Surface geology shows that the Riwat thrust is a foreland-vergent thrust along which lower to middle Siwalik molasse strata are thrust southward over upper Siwalik strata. Seismic reflec tion interpretation shows that the Rin-at thrust developed as a roof t hrust of a hinterland-vergent tectonic wedge (triangle zone) underlain by evaporites, The Riwat thrust propagates upsection from a depth of about 4 km at the base of the Siwalik Group. At this depth, it merges into a hinterland-vergent blind thrust that propagates upsection as a ramp from Eocambrian evaporites covering the basement at a depth of ab out 6 km. Bounded between this set of conjugate faults, a tectonic wed ge of Eocambrian (evaporites) to Neogene strata is thrust toward the h interland to form a triangle zone. The roof thrusts of triangle zones have been widely mapped as backthrusts in deformed mountain fronts. Hi nterland motion of tectonic wedges as in the Rin at thrust triangle zo ne may be a feature of the fold-and-thrust belts underlain by evaporit es acting as an extremely weak decollement layer. Their recognition, w ith a trap-forming geometry below a thrust, is important for interpret ing particular fold belts and for hydrocarbon exploration, These struc tures could be predicted by the surface geology data where hinterland vergence of a fold below a thrust is apparent; however, seismic reflec tion data appear to be critical in recognizing these structures.